1. Field of the Invention
The present invention relates to a printed sheet measurement apparatus for measuring a colorimetric value and the like on a printed sheet based on image data obtained by capturing an image on the printed sheet to effect color control of printed sheets.
2. Description of the Background Art
Conventionally, a printed sheet measurement apparatus has been known in the art which captures an image on a printed sheet placed on a table to measure a colorimetric value in any position on the printed sheet based on the resultant image data. Such a printed sheet measurement apparatus measures the colorimetric value in a predetermined position on a printed sheet sampled from a printing machine to control the amount of ink supply in the printing machine in accordance with a result of comparison between the measured colorimetric value and a colorimetric value in its corresponding position included in preset reference image data. It is contemplated that the reference image data is obtained, for example, by capturing an image on a previously prepared reference printed sheet (which is a printed sheet or a printed sample produced by proof printing and referred to hereinafter as a reference sheet).
Measuring the colorimetric value (or color density), as that term is used herein, refers to converting a color on the printed sheet into a numerical form represented in a predetermined color system. As an example, the term refers to determining a density value corresponding to each color of YMCK or RGB and a numerical value represented in a known color system such as L*a*b* color system.
However, the reference sheet is not always in the same coordinate relationship with the sampled printed sheet. Specifically, there is a difference, in some cases, in size, position or appearance between the reference sheet and the sampled printed sheet.
The difference in size between the reference sheet and the sampled printed sheet arises, for example, when the previously prepared reference sheet is used in a prior print job and the print size is scaled in the current print job or when the proof printing is performed on the reference sheet different in size from the sampled printed sheet because of the limited size supported by an apparatus itself for performing the proof printing. Further, there are cases where the printed sheets themselves have slightly different sizes.
The difference in position between the reference sheet and the sampled printed sheet arises, for example, when misregistration occurs during the image capture by the printed sheet measurement apparatus.
The difference in appearance between the reference sheet and the sampled printed sheet arises, for example, when the sampled printed sheet is subjected to imposition. In the printing process for a book or the like, a sheet including a plurality of imposed pages is printed, and is then subjected to a predetermined folding process and a bookbinding process. Thus, the sampled printed sheet includes the plurality of imposed pages. Since the plurality of imposed pages are arranged depending on how to fold the sheet or so as to properly form a signature, the plurality of pages are not arranged in consecutive order, and some of the individual pages are inverted. However, a proof sheet obtained when a printed sample is produced sometimes consists of a single page or a pair of pages which become facing pages after bookbinding because of the limited output size of a proof system. In other cases, a printed sample is prepared and presented by a client.
The above-mentioned difference in coordinate relationship between the reference sheet serving as a color matching reference and the actual printed sheet brings about nonequivalent corresponding positions on the image data about the reference sheet and the actual printed sheet which are produced by the printed sheet measurement apparatus. Therefore, the background art printed sheet measurement apparatus might give rise to nonequivalent positions of measurement of the colorimetric values to be compared with each other, thereby to fail to precisely effect the color control of the printed sheets.